Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/52—Devices using data or image processing specially adapted for diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/5207—Devices using data or image processing specially adapted for diagnosis using ultrasonic, sonic or infrasonic waves involving processing of raw data to produce diagnostic data, e.g. for generating an image
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/0048—Detecting, measuring or recording by applying mechanical forces or stimuli
  • A61B5/0051—Detecting, measuring or recording by applying mechanical forces or stimuli by applying vibrations
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/72—Signal processing specially adapted for physiological signals or for diagnostic purposes
  • A61B5/7235—Details of waveform analysis
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/44—Constructional features of the ultrasonic, sonic or infrasonic diagnostic device
  • A61B8/4483—Constructional features of the ultrasonic, sonic or infrasonic diagnostic device characterised by features of the ultrasound transducer
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/48—Diagnostic techniques
  • A61B8/485—Diagnostic techniques involving measuring strain or elastic properties
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B2562/00—Details of sensors; Constructional details of sensor housings or probes; Accessories for sensors
  • A61B2562/02—Details of sensors specially adapted for in-vivo measurements
  • A61B2562/0204—Acoustic sensors
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
  • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/42—Detecting, measuring or recording for evaluating the gastrointestinal, the endocrine or the exocrine systems
  • A61B5/4222—Evaluating particular parts, e.g. particular organs
  • A61B5/4244—Evaluating particular parts, e.g. particular organs liver

Definitions

• the present invention is directed to detecting shear-wave time -of- arrival and, more particularly, to doing so based on shear-wave-induced displacement.
• ultrasound shear wave elastography One method for measuring tissue mechanical properties is ultrasound shear wave elastography. It utilizes acoustic radiation force (ARF) to generate shear waves in soft tissue and subsequently tracks shear wave displacement to estimate tissue elasticity and viscosity.
• ARF acoustic radiation force
• An application of this technique is the non-invasive measurement of liver stiffness to stage liver fibrosis and cirrhosis.
• Interrogation by ultrasound for purposes of medical imaging, often makes use of longitudinal waves.
• the ultrasound propagates in wave form.
• particles all along the propagation path vibrate, in place, back and forth, the vibration occurring in the direction of propagation.
• the vibrations create compressions and rarefactions. These are modeled as the peaks and valleys of a sinusoid. Energy is conveyed to the target and back by means of the oscillatory particle movements.
• An ultrasound shear (or transverse) wave is characterized by back and forth in-place movement that is perpendicular to the direction of propagation. Oscillation one way creates the peaks, and the other way creates the valleys.
• the wave is comprised of components, each oscillating at its own frequency. It is the propagation speed of the wave envelope, or "group velocity", which is sought.
• group velocity the propagation speed of the wave envelope
• a focused longitudinal-wave push pulse is issued. It is a high intensity, long duration and narrow bandwidth signal.
• the push pulse creates a shear wave.
• the focal depth has been selected, at the outset, so that the shear wave travels through a region of interest (ROI). Push pulses can be fired repeatedly for multiple measurements to increase accuracy of the estimation of shear wave velocity.
• a typical repetition rate is 100 Hz.
• a longitudinal-wave tracking pulse is issued to the ROI to assess, at the sampling point (or “lateral location"), the amplitude of the shear wave over a certain observation period (on the order of 10 ms).
• the measurement is of the body-tissue displacement perpendicular to the lateral direction of shear-wave propagation, that propagation being away from the region of excitation (ROE) at the push focus.
• the time period between the peak displacement and the push pulse responsible is called the time-to-peak (TTP).
• TTP time-to-peak
• An example of the TTP technique is found in U.S. Patent Publication No. 2008/0249408 to Palmeri et al. (hereinafter "Palmeri”), the disclosure of which is incorporated herein by reference in its entirety.
• the TTP is similarly determined for a number of lateral locations located, in mutual linear alignment along the propagation path, outward from the ROE. Time delays between peaks of different locations can be derived, as in acoustic radiation force impulse (ARFI) imaging.
• ARFI acoustic radiation force impulse
• a functional relationship can be estimated between distance the shear wave propagates and the time over which the distance is traversed.
• TTP assumes that a wave arrives at, or passes over, a lateral location when the peak displacement of the wave occurs at that location. It further assumes that the peak displacement outside the ROE travels at the group velocity of the shear wave.
• wave-arrival time at a location can be characterized by the time at which the center of mass (COM) of the in-situ waveform exists at that location, and that the COM technique tends to yield shear-wave velocity values closer to ground truth than the TTP technique, especially for tissue with high shear modulus and/or shear viscosity. This is mostly applicable when lateral locations are close to the ROE, to yield the best signal-to-noise ratio (SNR). With the locations close to the ROE, dispersion will have more impact on TTP than on COM, thus making COM desirable. The inventors have further observed that COM is less prone to errors from multi-peaks when SNR is low.
• a weighted average of sampling times is utilized to estimate the time corresponding to the COM of the waveform particular to the lateral location at which the sampling occurs in the body tissue under examination.
• the sampling can be performed at multiple lateral locations based on the same push.
• the push cycle may be repeated with different timings of the sampling to "fill out" each waveform.
• the sampling-time factors of candidate summands, or the summands, of the weighted average can be selectively excluded if the displacement observed at the sampling time fails to exceed a threshold.
• Summands/factors that survive this first stage of filtering can also be required to exhibit a continuous displacement segment around peak and above a peak-neighborhood threshold.
• the segment can correspond to a curve fitted to the series of consecutive displacement values, and be deemed continuous if, for example, the sum of absolute distances of the values from the curve does not exceed a summed-deviation threshold.
• lateral locations can likewise be selectively excluded based on sufficiently low displacement.
• the low displacement in either case, can allow noise to assume a greater adverse impact. Accordingly, low signal-to-noise ratio (SNR) data is filtered out of the wave-arrival time computation, thereby increasing its accuracy.
• SNR signal-to-noise ratio
• a device is designed for using shear- wave-induced displacement measured at a location along a propagation path to compute a weighted average representative of arrival time of a shear wave at the location.
• the device in one aspect, is further configured for estimating shear-wave propagation speed based on a time the shear wave is present at a different location along the path.
• the speed is a magnitude of group velocity of the shear wave.
• the weighting is based on the displacement.
• the weighting is by the displacement.
• the values weighted correspond to times associated with sampling that detects the displacement.
• the displacement is, based on whether it meets an instantaneous-displacement threshold, selectively excluded from computation of the average.
• the device is configured for the using, of the displacement, at a plurality of locations, the threshold varying with the location for which the respective average is being computed.
• the threshold is based on peak displacement of material a medium comprises, the path being through the medium.
• the threshold is directly proportional to the peak displacement.
• the device is configured for the using at a plurality of locations to compute respective weighted averages representative of respective arrival times.
• the threshold is equal to the peak displacement multiplied by a factor invariant with location. The peak displacement varies with location
• the factor dynamically varies based on a criterion.
• the criterion is based on a noise metric.
• the device is configured for the using at a plurality of locations and for selectively excluding one or more of the plural locations.
• the device is further configured such that the selecting is dynamically based on a criterion.
• the criterion includes whether respective peak displacement exceeds a peak-displacement threshold.
• the device includes an ultrasound transducer configured for the measuring.
• the device is implemented as one or more integrated circuits.
• the weighted average is equal to the arrival time at the position.
• the displacement is displacement of body tissue.
• the method for generating comprises varying an electrical current applied to at least one of: a) a wire input to said device; and b) an antenna for transmitting, so as to, by the varying, generate the signal.
• FIG. 1 is a schematic diagram exemplary of an ultrasound probe making measurements from a medium
• FIG. 2 is an exemplary series of location-based graphs of displacement over time, and shows formulas used in shear-wave velocity determination
• FIG. 3 is a flow chart illustrative of system operation
• FIG. 4 is a schematic, transmit-receive diagram illustrating, by example, sample acquisition using retrospective dynamic transmit (RDT) as implemented on a 16x multiline beamformer; and
• FIG. 5 is a schematic diagram demonstrating, in an RDT context, a possible placement of a detection beam transmit focus.
• FIG. 1 is an example of an ultrasound probe 100 comprising an ultrasound transducer (not shown) firing a pushing beam 104 into a medium 108 such as body tissue.
• a medium 108 such as body tissue.
• high intensity, narrow bandwidth signals are fired with a repetition rate of, for example, 100Hz.
• the probe 100 includes an in-place, displacement-based weighter 1 10.
• the resulting pushing beams 104 each cause a respective shear wave 112.
• tracking beams 116, 120, 124, 128, 132 issue from the ultrasound probe 100, and their respective echoes (not shown) are received for processing.
• the tracking beams 1 16-132 are utilized to measure shear- wave-induced vertical displacement in the body tissue 108 at different spatial locations 136, 140, 144, 148, 152 along a path 156 of the shear wave 112. Initially, reference pulses (not shown) are issued to each of the locations to provide a frame of reference for subsequent measurement of the displacement.
• the shear wave 112 travels through soft body tissue at a speed that is roughly a thousand times slower than the 1540 meter (m) per second (s) at which an ultrasound wave propagates.
• the displacement measured at a location 136-152 is, in accordance with what is proposed herein, utilized by the weighter 1 10 to determine when the shear wave 1 12 has arrived at the location, as explained in more detail further below. These determined times are usable to calculate a mechanical parameter of the tissue from which the displacement measurements were made. Examples of such mechanical parameters are shear elastic modulus, Young's modulus, dynamic shear viscosity, shear wave velocity and mechanical impedance.
• a functional relationship can be estimated between location-to-location distance the shear wave 112 propagates and the time period during which the inter- location propagation occurs.
• Linear regression can be used for making the determination.
• the slope of the regression line is indicative of the magnitude of the group velocity of the shear wave 112.
• the magnitude is the shear-wave propagation speed commonly used to calculate the shear elastic modulus of the tissue 108.
• a clinical determination regarding the tissue 108 can then be made.
• the probe 100 or an apparatus that includes the probe, is implementable as the device of claim 1 for, by way of example, judging disease status or lesion malignancy.
• control circuitry serving as the device of claim 1 can take the form of one or more integrated circuits (ICs).
• ICs integrated circuits
• One or more ICs in accordance with claim 1 can, alternatively, be configured for installation into existing ultrasound machines to enhance the
• Shear waves are described herein above as being created by ARF which is noninvasive, but can alternatively be generated by coupling an external mechanical source to the site of interest. Imaging of the displacement can be performed using magnetic resonance imaging (MRI) instead of ultrasound, but MRI is relatively expensive and takes longer.
• MRI magnetic resonance imaging
• FIG. 2 shows, by way of illustrative and non-limitative example, five
• displacement-versus-time graphs or "displacement curves", 206, 210, 214, 218, 222. They correspond to the five locations 136-152, respectively.
• the locations 136-152 are near the probe pushing focus, i.e., at the end of the pushing beam 104. They are disposed along the propagation path 156 in a line perpendicular to the axis of the probe 100, and they are located at the same depth as the probe pushing focus.
• Displacement 226 is in micrometers ( ⁇ ) and time 230 is in milliseconds (ms). The spacing between successive locations 136-152 is 0.5 mm.
• the weighter 1 which is illustratively shown operatively connected with the fourth tracking beam 128 but which likewise is connected to all the tracking beams 1 16- 132, obtains the COM for each location 136-152 by taking a weighted average of the respective curve 206-222.
• the weighting is by displacement.
• the values being weighted correspond to times associated with sampling that detects the displacement.
• a time value 230 of 1 ms would be weighted by the displacement 226 of 10 ⁇ .
• all, or some subset, of the values 230 could individually be weighted according to displacement 226, and added to form a sum 238.
• "disp" stands for displacement 226.
• the variable "t” stands for the time 230 at which displacement is sampled.
• "Peak” stands for peak displacement 239 and "H” stands for instantaneous-displacement threshold factor 240, as discussed further below.
• the sum 238 is divided by a sum 242 of the displacements 226 used in the weighting, thereby forming a weighted average 244 represented by the symbol tco M.
• the parameter, tco M is, for the given location 136-152, the COM position in the temporal domain, and is equal to the weighted average 244 computed.
• the parameter tco M represents the time of arrival of a shear wave 1 12 at the location 136-152, each location having its respective tco M -
• a propagation speed 246 is obtainable by the above-mentioned regression technique based on known distances 250 between locations 136-152 and the time period 254 between respective shear-wave time-of-arrivals 244, i.e., respective parameters tco M - This is shown in FIG. 2, where Ad stands for the distance between two locations 136-152, and AtcoM stands for the difference between the two respective weighted averages 244 represented by the symbol tcoM.
• the displacement curves 206-222 are shown as strictly positive -valued, the curve for any given location 136-152 may alternatively be strictly negative -valued, as discussed further below. Locations 136-152 for which the displacement curve 206-222 is a crest are comparable to each other for the purpose of calculating the propagation speed 246; likewise, locations for which the displacement curve is a valley are comparable for same purpose.
• noise may be filtered from the displacement data, thereby making the weighted average 244 more robust.
• Some locations 136-152 may involve more noise.
• the first two locations 136, 140 may be disposed within the push beam 104 so that pure shear wave propagation is no longer a valid assumption between these two locations. This depends on many factors, such as transducer frequency, f number, focal depth and tracking spacing. Also, to penetrate to a deeper region of interest (ROI), a lower frequency of ultrasound is needed. The consequent longer wavelength sacrifices anatomical detail, thereby decreasing signal-to-noise ratio (SNR).
• SNR signal-to-noise ratio
• Patient respiration and heartbeat involve movement that can decrease SNR. If a tumor is being imaged, for instance, it may be closer or further from the heart or a major artery, resulting in more or less noise.
• the sampling can be time-gated to avoid these motion artifacts.
• the noise can vary with the quality or type of ultrasound equipment used. For example, a scanner of reduced size or reduced transducer element size may produce more noise.
• the noise has a greater adverse impact on smaller displacement-data used in the computation of arrival time 244.
• the locations 136-152 that are to be used in the computation of arrival time 244 are selected.
• the first location 136 may selectively be excluded, as discussed herein above, for not reliably exhibiting pure shear-wave propagation.
• the exclusion, if any, by location can be pre-fixed, where, for example, the first location 136 is excluded.
• Exclusion can, alternatively or in addition, be dynamically determined based on certain criteria, such as peak, i.e., maximum, displacement having to be greater than a pre-set peak-displacement threshold.
• an instantaneous-displacement threshold varies by displacement curve 206-222 as the cutoff above which data can be trusted.
• the threshold can be based on peak displacement 239. It, for example, may be set equal to peak displacement multiplied by the instantaneous-displacement threshold factor 240 that is invariant with location.
• the factor 240 may be pre-fixed, e.g., at 50%. Or, it can be dynamically selected based on certain criteria.
• the criteria can entail a noise metric, such as SNR.
• location exclusion may be pre-set by the operator, specifying which locations will be processed or excluded (step S310). Likewise, the operator can, if it is variable, enter the peak-displacement threshold, and can do the same for the instantaneous-displacement threshold factor 240 (step S320). After the sampling has been performed and subject to application of the inputs from the above steps S310, S320, dynamic location-exclusion is made according to the criteria set (step S330). In addition, displacement 226 that does not meet the instantaneous-displacement threshold is filtered out, i.e., not used in the computation of arrival time 244 (step S340).
• a single wavelength passes by a given location 136-152 in 10 ms.
• a tracking frequency i.e., tracking pulse repetition rate (PRF)
• PRF tracking pulse repetition rate
• 100 samples could be taken if not for the time expended for the push 104.
• slightly fewer than 100 samples e.g., 95 samples, can be taken during the 10 ms.
• the locations 136-152 can be successively sampled in a concurrent manner.
• the first sample is at the first location 136
• the second sample is at the second location 140, and so on.
• One pass consisting of five samples, one sample per each location 136-152, can be repeated 19 times, for example, for a single preceding push 104.
• each weighted average 244 can be based on 19 weighted values.
• the 19 values per location 136-152 can be supplemented with time -wise intermediate samples by observing displacement 226 over a number of pushes. With each subsequent push, the order of sampling is changed.
• the first sample may be at second location
• the second sample may be at the third location, and so on, with the fifth sample being at the first location.
• the sampling is shifted location-wise by one more location, and so on.
• each of the locations 136-152 has been sampled at 95 equally- spaced time intervals that span the lifetime of the crest, or valley, or a portion thereof being sampled.
• the weighted values of a single time-of-arrival computation 244 are based on sampling of temporally-different shear waves.
• estimating shear- wave propagation speed is based on times the shear wave 1 12 is present at different locations 136-152, as gauged from data acquired from shear waves emanating from different pushes 104 that is used in the same calculation.
• the sampling order of the locations 136-152 can be kept constant, with the onset of sampling after each uniform push 104 delayed, further each time, by an inter-sampling time period.
• the locations 136-152 are sampled in order, for 19 passes.
• the second push 104 the same sampling order is retained, but the onset of sampling after the push is delayed.
• the delay i.e., inter-sampling time period, is equal to the time between successive samples.
• the first location 136 is sampled at a time-since-push that is slightly greater after the second push 104 than it was after the first push.
• each of the locations 136-152 has been sampled at equally- spaced time intervals that span the lifetime of the crest, or valley, or portion thereof being sampled.
• the missing data would likely be filtered out anyway for not meeting the instantaneous-displacement threshold. The missing data could also be interpolated back, and then be subject to the filtering.
• a multiline receive scheme collects more information per each tracking pulse. From each tracking or "transmission" pulse (or “transmit”), multiple receive lines are formed. Palmeri uses 4 in-parallel-directed receive lines, although more, such as 16 or 32 could be used. The receive lines are dynamically formed and spatially parallel.
• multiline -receive sample acquisition can be performed using retrospective dynamic transmit (RDT).
• RDT retrospective dynamic transmit
• Shear- wave sampling context in the commonly-assigned patent application, based on Philips Invention Disclosure 776394, entitled “Spatially-Fine Shear Wave Dispersion Ultrasound Vibrometry Sampling" to Burcher et al. (hereinafter "Burcher”).
• the Burcher patent application describes how a cycle of four spatially-staggered transmit and receive apertures can sample four spatial locations on a shear wave.
• eight spatial locations can be densely sampled at, for example, a spacing of merely 0.125 mm. Close inter- location spacing affords sampling closer to the push 104. This is beneficial in avoiding degradation of the measurements made of a shear-wave envelope whose magnitude perpendicular to the propagation axis decays over time.
• FIG. 4 depicts an exemplary RDT scheme implemented on a 16x multiline beamformer, i.e., a beamformer that forms 16 receive lines from one transmit beam (or "transmit" for short).
• sampling is by group of spatial locations.
• a transmission beam is issued to each group. If the ROI is located at a different depth than the focus of the transmission beam, then the transmission beam will be broader than at the focus and will insonify the group of spatial locations.
• the beam can be weakly focused at the same depth as the ROI, with a breadth that is sufficient to insonify the group of spatial locations.
• 16x beamforming circuitry From the echoes of a transmission beam created from a single tracking pulse 404, 16x beamforming circuitry forms 16 in -parallel-directed receive lines 411-426 for making measurements of the shear wave 1 12. As indicated by a diagonal, sub-aperture tracking line 427, the first eight receive lines 411 -418 are on one side of the center of the transmit, and the second eight receive lines 419-426 are on the other side.
• Subsequent tracking pulses 428, 429, 430, 431 and the first tracking pulse 404 are all differently-timed. If the transmission A-line (or "tracking pulse") PRF is set equal to 10 kHz for example, the single tracking pulse 404 issues 100 ⁇ & before the next tracking pulse 428. 100 ⁇ & later, the next pulse 429 fires, and so on.
• Each of the tracking pulses 404, 428, 429, 430, 431 is formed by a respective then-currently active sub-aperture of the tracking probe.
• the active sub-aperture is a subset of the transducer elements that are currently active to transmit ultrasound.
• the currently active sub-aperture shifts (e.g., one or more elements on one side of the aperture are excluded and one or more elements on the other side are included).
• the next tracking pulse 428 then fires, it occurs from the shifted sub-aperture.
• the spatial distance between adjacent apertures is referred to herein after as a transmit spacing 434, which, in the current example, is 0.5 mm.
• the focal point of the tracking pulse also shifts by the transmit spacing 434 between consecutive tracking pulses.
• the 16 in-parallel-directed receive lines 411 -426 i.e., dynamically- formed receive lines that are spatially parallel
• Each of the receive lines 411 -426 is formed by a receive sub-aperture.
• the receive sub-aperture is a subset of the transducer elements that contribute to a given receive line.
• the spatial distance between the receive lines 411 -426 is referred to hereinafter as a receive spacing 438. In this example, it is 0.125 mm, or one quarter of the 0.5 mm transmit spacing 434.
• All of the acquired echo radiofrequency data is saved in temporary storage. Retention of acquired data will continue as the sub-aperture shifts and eventually assumes its final position in the transducer array, i.e., so that an entire pass of data is acquired. In addition, data will be retained pass-to-pass.
• the last twelve receive lines 415-426 of that first pulse spatially overlap, respectively, with the first twelve receive lines of that next pulse.
• the last twelve receive lines of the tracking pulse 428 overlap with the first twelve receive lines of the next tracking pulse 429, and so on.
• respective receive lines of all five tracking pulses 404, 428, 429, 430, 431 overlap and can be combined to form eight reconstructed A-lines corresponding to the eight locations 451 -458.
• the first reconstructed A-line for the location 451 is formed from the first receive line 423, combined with the three respective receive lines of the immediately-subsequent tracking pulses 428-430, all four receive lines being aligned with the location.
• the combining occurs in accordance with retrospective dynamic transmit (RDT).
• RDT retrospective dynamic transmit
• the effect of RDT focusing can be analyzed using the virtual transducer approximation proposed by Passmann and Ermert in 1996. See C. Passmann & H. Ermert, "A 100-MHz ultrasound imaging system for dermato logic and ophthalmologic diagnostics," IEEE Trans. Ultrasonics, Ferroelectrics and Frequency Control, vol. 43, no. 4, pp. 545-52 (1996).
• the first reconstructed A-line for the location 451 is for measuring the shear wave 112 at a spatial location 451.
• the immediately-subsequent reconstructed A-lines for the locations 452-458 which are laterally offset from the first reconstructed A-line for the location 451 , are for measuring the shear wave 1 12 at the respective locations 452-458.
• the aperture shift may be to an extent that more or fewer receive lines are combinable to form a reconstructed A-line.
• DOF depth of field
• SNR signal-to-noise ratio
• A-line reconstruction based on potentially four receive lines begins with the first four reconstructed A-lines for the locations 451-454, and proceeds with each new tracking pulse.
• the next tracking pulse allows for the formation of four new reconstructed A-lines for the locations 455-458.
• each succeeding tracking pulse leads to the formation of a respective plurality of reconstructed A-lines, that plurality being, in the current example, made up of four reconstructed A- lines, in the event more locations are to be sampled.
• 19 passes can be made following the push 104.
• data acquisition can entail multiple, e.g., 5, pushes.
• the locations can be sampled, as described above, with the onset of sampling after each push 104 delayed, to thereby enhance the weighted average calculations.
• the shear wave 1 12 is therefore finely sampled, without a reduction in the pace by which tracking pulses issue.
• RDT When RDT combines transmits to interpolate intermediate transmit locations (as disclosed in the '693 publication), the sampling time (as well as the sampling location) is interpolated between transmits. In other words, in the case of pulse firing times, and in the case of the transmit locations upon firing (e.g., those along the sub-aperture tracking line 427 for transmits used in the reconstruction), they are interpolated by the same interpolation weightings used in RDT A-line reconstruction in the '693 publication.
• FIG. 5 demonstrates, in an RDT context, a possible placement of a focus 504 of a detection beam 508 formed by the tracking pulse 404.
• a region of interest (ROI) 512 is where a shear wave 516 is present.
• ROI region of interest
• a physical focus position shown in FIG. 5 is at 70 mm, a common depth 518 for transmits insonifying a given point in the ROI and therefore to be RDT- combined.
• the RDT reconstructed detection beam will, at that depth, be narrow - having the same width as at the (physical) focus 504.
• the temporally-initial tracking pulse 404 from among the temporally-initial tracking pulse 404, and at least one laterally-offset tracking pulse 428- 431 forming a transmit to be RDT-combined with that of the temporally-initial tracking pulse, at least some are focused to a common depth 518.
• a shallower transmit focus, to the depth 520 is reconstructed based on echo data from those of the pulses 404, 428-431 focused to the common depth 518.
• the focus 504 of the detection beam 508 may be placed shallow to the ROI 512.
• RDT enjoys the above-noted advantages, despite its assumption that the tissue does not move between transmit events. If displacement does occur, then it will reduce the coherence between the combined transmits and lead to signal cancellation. This would therefore appear to be incompatible with the present technique, since the tissue is being displaced by the shear wave.
• the displacements 226 are of such small magnitude (typically ⁇ 20 ⁇ ) that this is a small fraction of the wavelength of the tracking pulse (e.g. 300 ⁇ at 5 MHz). Therefore, the shear wave displacements 226 will not cause any significant loss in coherence during the transmit reconstruction.
• the lateral locations be close to the excitation point, i.e., the push focus, to avoid attenuation of the shear wave amplitude which makes it hard to measure the amplitude, although the first lateral location is limited in its closeness to the excitation point, due to near- field effects.
• Sufficiency of SNR may serve as a guide on how far away from the excitation point the lateral locations are to be placed in order to avoid the near- field artifacts. Closeness between adjacent locations is likewise desired to avoid attenuation, but is constrained by other engineering considerations.
• the weighted-average-based position in the temporal domain is computed based on the sampling of shear wave displacement along the propagation path.
• the weighting is, for example, by displacement observed at times corresponding to sampling and represents the time of arrival of the shear wave at the shear-wave propagation path location being sampled.
• the computed shear-wave times of arrival at respective locations are functionally related to known inter-location distances to derive shear-wave group velocity.
• the derived velocity can serve as input into known algorithms for estimating shear elasticity of the medium, such as body tissue, for purposes of clinical diagnosis and therapy assessment.
• all samples e.g., 95
• all samples can be acquired for a single, particular location following its push, and then, following a next push, 95 for a next location, and so on.
• the push-to-onset-of-sampling time would be the same for all locations.
• the sampling at different locations would correspond to the same, established waveform.
• a computer program can be stored momentarily, temporarily or for a longer period of time on a suitable computer-readable medium, such as an optical storage medium or a solid-state medium.
• a suitable computer-readable medium such as an optical storage medium or a solid-state medium.
• Such a medium is non-transitory only in the sense of not being a transitory, propagating signal, but includes other forms of computer-readable media such as register memory, processor cache and RAM.
• the 100 and for conveying it to the device, is formable by appropriately varying an electrical current.
• the signal can arrive by a device input wire, or be transmitted wirelessly by an antenna.
• a single processor or other unit may fulfill the functions of several items recited in the claims.
• the mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

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• 2011
  • 2011-12-19 WO PCT/IB2011/055768 patent/WO2012085812A2/en active Application Filing
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  • 2011-12-19 EP EP11813818.9A patent/EP2654552B1/en active Active
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